Extrusion of thermoplastic polymer compositions is a well known method for mass producing finished or semi-finished articles. Conventional thermoplastic compositions for extrusion processing contain a thermoplastic polymer, generally as the main constituent, and inorganic and/or organic fillers. Wood flour is often employed as the organic filler. Wood-filled thermoplastic compositions can be extruded or injection molded into articles which are relatively lighter in weight and less expensive than similar mineral filled thermoplastic articles made without the wood flour. At the same time, such wood-filled articles have certain desirable physical properties which are the same as or, generally, better than those of articles made from similar thermoplastic materials not containing the wood flour. Therefore, wood-filled thermoplastic compositions are employed to make articles suitable for a variety of uses.
U.S. Pat. No. 4,203,876 to Dereppe et al teaches a moldable composition consisting essentially of thermoplastic polymers, vegetable materials such as wood flour and a synthetic elastomer. It is suggested therein that the moldable compositions can be used to manufacture cases of crates intended for the storage and transportation of perishable products such as fruits and vegetables.
In U.S. Pat. No. 4,228,116 to Colombo et al., a method for producing remoldable panels is taught. The method comprises forming a particulate mixture of thermoplastic polymer and particulate organic filler, e.g., wood particles, plastifying and extruding the mixture into a web, and rolling the web, which may subsequently be cut into panels. Colombo et al suggest that the plastic panels may be used as is or they may be reheated and pressed into three dimensional shapes, e.g., into furniture and dashboards or other components for automobiles.
U.S. Pat. No. 4,480,061 to Coughlin et al. teaches articles having a wood-like appearance and made from a composition comprising cellulosic materials, e.g., wood flour, ethylene interpolymers and processing oil. The compositions are suggested useful for molding into articles having a wood-like appearance such as picture and window frames.
Current rigid, wood-filled, thermoplastic articles have inherent deficiencies however which limit their usefulness in automobiles. For example, they tend to be somewhat brittle and thus if used in an area where they are difficult to install, they may crack during installation. Still further, they have less than desirable sound-deadening properties. Such articles, if intended for use in motor vehicles, should desirably be sound absorbing/reflecting, i.e., sound-deadening. In an automobile, noise can enter from outside the vehicle, e.g., road or tire noise, from the trunk due to the collision of objects therein, or can be that vibrational noise transmitted to structural components inside the vehicle from the engine or drive train. Such transmitted vibrational noise may also be picked up by the automobile components as a sympathetic vibration and is termed "drumming". If a thermoplastic wood-filled article were to be used in an automobile as, e.g., a package tray positioned between the trunk and the passenger section of the automobile, it must also be capable of being sound-deadening. This means that it desirably is capable of absorbing noise travelling in from the trunk, of absorbing road noise, and further that it does not pick up sympathetic vibrations from sound impinging on it through adjoining structures, i.e., it does not "drum".
In order to improve the sound-deadening properties of currently employed wood-filled thermoplastic articles (e.g., automobile package trays), they are encased (sandwiched) between thick sound-absorbing materials such as sound-absorbing urethane, shoddy/cotton or other fiber mat. The use of various sound-absorbing sheeting materials in automobiles is well known in the industry and in patent literature. For example, in U.S. Pat. Nos. 4,191,798, 4,403,007, 4,430,468, 4,434,258 and 4,379,190, ethylene based sheeting materials are disclosed to be useful for their sound-deadening properties and may be employed as automobile carpet backings. U.S. Pat. No. 3,904,456 to Schwartz, discloses self-supporting sound-deadening sheets of ethylene-vinyl acetate copolymer highly loaded with inorganic filler material. The sheets can be molded into, e.g., fender well sections or can be applied to a component of the automobile, e.g., a body panel, to inhibit the transmission of airborne noises. It is the use of high density inorganic filler in these sheeting materials which absorbs or reduces the sound transmission. However the manufacture of a finished article by sandwiching the thermoplastic material between layers of sound-deadening material requires complex production and assembly techniques. It would be preferable if a finished article could be made from a thermoplastic composition which had good sound-deadening properties of its own. Finished articles could be produced from such thermoplastic compositions encased within thinner sound-absorbing materials or none at all, while still having suitable sound-deadening properties. Such a composition would thus allow for the production of wood-filled thermoplastic finished articles, having sound-deadening properties, into crisp and more complex three dimensional shapes.
It is thus highly desirable to formulate a thermoplastic composition which is extrudable and subsequently remoldable into articles having sound-deadening properties. It is additionally desirable that such an article has limited flexibility, yet at the same time, is tough and impact resistant, i.e., it is be rigid without being brittle, so as to allow installation of the article without cracking. However, the fact is that if a conventional thermoplastic wood-filled article is to be employed in an automobile and it is to have sound-deadening properties, it is still encased in thick layers of sound-deadening materials.
We have now found that inclusion of small crosslinked elastomeric rubber particles into a thermoplastic composition produces molded articles with the desired properties described above, i.e., they are rigid without being brittle, are impact resistant and tough and have excellent sound-deadening properties. These sound-deadening properties of the composition can be further enhanced by the inclusion of optional organic fillers like barium sulfate.
None of the references mentioned above, either those directed to wood-filled materials or those directed to the sound-absorbing sheetings, teach Applicants' composition comprising polyolefin resin filled with cellulosic particulate and crosslinked elastomeric rubber microparticles.